Brake unit with brake booster, two master cylinders for antilock and/or propulsion or slip control situated in common housing including pivotable rocker

ABSTRACT

A brake unit for a road vehicle with a hydraulic dual-circuit brake system having two master cylinders, the brake booster and elements for antilock and/or propulsion or slip control in a common housing. A brake booster drive cylinder which can be subjected to the outlet pressure, proportional to the pedal travel, of a brake valve, is arranged between static master cylinders. Coupled to the piston of the drive cylinder is a pivotable rocker, the arms of which are each supported on one of the master cylinder pistons via tappet. Assigned to at least one of the master cylinders is a regulating cylinder having an opposed piston which can be supported on the piston of the master cylinder and which, when subjected on one side to the outlet pressure of the auxiliary pressure source of the brake booster, causes the piston of the master cylinder to be returnable towards its basic position counter to the actuating force exerted on it. Instead of a single acting counter cylinder, there can also be provided a double acting cylinder as regulating cylinder which can be used both for antilock control and for propulsion or slip control.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a brake unit for controlling the brakepressures in the wheel brakes of a road vehicle, the brake system ofwhich is designed as a dual-circuit brake system with a hydraulic brakebooster and which is equipped with an antilock system.

A brake unit of this type is known from German Offenlegungsschrift No.3,015,729.

In this brake unit, two jointly actuable master cylinders, each assignedto one of the brake circuits of the vehicle, are arranged next to oneanother in a twin design, in order to achieve a shorter overall length,as compared with a tandem master cylinder. The master cylinders aredesigned as step pistons, the smaller piston steps of which limit theoutlet-pressure spaces and the larger piston steps of which limit drivepressure spaces, into which can be fed an outlet pressure, proportionalto the pedal travel or to the pedal force, of a brake valve, by whichthis pressure utilized for brake boosting is derived from anauxiliary-pressure source. The outlet-pressure spaces of the mastercylinders and their drive pressure spaces are connected to one anotherby central channels passing through the master cylinder pistons in theaxial direction. Thus, their piston steps of smaller diameter, limitingthe outlet-pressure spaces, and their piston steps of larger diameter,limiting the drive pressure spaces, are respectively subjected to theoutlet pressure of the brake valve. Brake boosting takes place as aresult of the effect of this outlet pressure on the excess pistonsurfaces of the larger piston steps. Thus, when the auxiliary-pressuresource is intact, the two brake circuits operate as a dynamic brakecircuit.

In the event of failure of the auxiliary-pressure source, when the brakesystem is actuated, tappets, on which engages a yoke coupled to thebrake pedal, are forced against valve bodies, and these are forcedagainst valve seats, located on the same side as the outlet-pressurespaces, of the master cylinder pistons. The connection between the drivepressure spaces and the outlet-pressure spaces of the master cylindersis broken and the actuating force is transmitted directly, via thetappets, to the master cylinder pistons which are now displaceable bypedal force only. Thus, in the event of failure of theauxiliary-pressure source, the two brake circuits operate as staticbrake circuits.

For the antilock control, there are solenoid valves, in particularbrake-pressure regulating valves designed as 3/3-way solenoid valves.These valves can be commanded to move from a basic position, namely thepressure built-up position, in which the outlet pressure of a mastercylinder is fed into the respective connected wheel brake or wheelbrakes, through a blocking position into a pressure reducing position,in which the wheel brake or wheel brakes connected to the particularmaster cylinder are connected to the brake fluid storage tank of thebrake system and can be relieved towards this. This type of antilockcontrol, using the so-called bleeding principle, is necessary becauseunder normal circumstances, that is to say when the auxiliary-pressuresource is intact, the brake circuits are operated dynamically.

If the brake-pressure regulating valves and the brake unit are combinedtogether into a hydraulic control unit on the housing block, this beingthe most expedient both in production terms and to keep the ductingshort, this constructional unit acquires a relatively largeconstructional volume, and this presents considerable problems when itcomes to accommodating it in the confined engine space of the vehicle.Another disadvantage is that an antilock control working on the bleedingprinciple exhibits a relatively sluggish response behavior at low brakepressures. That is to say when the control starts at very lowcoefficients of adhesion between the roadway and the vehicle wheelssubjected to the control, since the pressure drop critical for thepressure reduction rate, between a wheel brake subjected to the controland the brake fluid storage tank is correspondingly low.

The object of the invention is, therefore, to improve a brake unit ofthe type mentioned in the introduction for a vehicle with an antilocksystem, to the effect that a hydraulic control unit comprising the brakeunit and devices for antilock control can be produced withadvantageously small spatial dimensions, and increased sensitivity ofthe brake-pressure control resulting in a faster response behavior canbe obtained.

According to the invention, this object is achieved by the features tobe mentioned.

According to these, the master cylinders provided for brake pressurecontrol in the brake circuits I and II are designed as static cylindersactuated by a drive cylinder which engages master cylinder pistons via apivotable rocker. This results in a uniform transmission of theactuating forces to the master cylinder pistons being achieved accordingto a predetermined brake pressure distribution to the brake circuits.The hydraulic drive cylinder provided for driving these can be arrangeddirectly, including the brake valve of the brake booster, between themaster cylinders of the brake unit, without contributing to an increasein the overall length of the brake unit. The use of static mastercylinders which allow brake pressure control on the principle of volumevariation, namely enlargement for pressure reduction phases anddiminution for pressure build-up phases in antilock control, incombination with the transmission of the actuating forces to the mastercylinder pistons via a pivotable rocker, makes it possible to vary thebrake pressure in one of the two brake circuits as a result of thedisplacement of the piston of its master cylinder, without anyappreciable reaction on the other brake circuit occurring thereby. Onthe contrary, the other brake circuit remains actuated with the forceselected via the brake valve and the drive cylinder. A counter cylinderprovided for the displacement of the master cylinder piston of the brakecircuit subjected to the control and the piston of which is coupled interms of movement to the master cylinder piston can be incorporated in asimple way into the brake unit, for example in a lateral arrangementnext to the particular master cylinder, without the overall length ofthe brake unit being increased. The activation of the regulatingcylinder provided for the control by the high outlet pressure of theauxiliary-pressure source ensures that the control responds quickly and,in particular, even at low absolute values of the brake pressure, apressure reduction phase of the antilock control can take place at ahigh pressure reduction rate.

The same also applies accordingly when the brake unit according to theinvention is additionally designed for controlling pressure build-up andpressure reduction phases of a starting-slip control.

A regulating cylinder suitable both for the antilock control and for theslip/propulsion control is then advantageously designed as adouble-acting drive cylinder.

The features also provide a constructionally simple design of the pistonof a master cylinder of the brake unit which is assigned to a brakecircuit which can be subjected both to antilock control and toslip/propulsion control.

The features also provide arrangements and simple designs of regulatingcylinders for antilock control which ensure that the regulating forcesare transmitted free of pull-out torque to the master cylinder pistonsand which can be incorporated in the brake unit, with only a smallamount of space required.

The same applies accordingly to the basic construction and alternativedesign of a counter-cylinder suitable for the antilock control, with anannular piston arranged coaxially relative to the drive cylinder.

The features also provide a simple design and arrangement, suitable incombination with the above-mentioned regulating cylinders for anantilock control, of a regulating cylinder which is suitable for anadditional slip/propulsion control, but which requires only a slightincrease in the overall length of the brake unit.

Such an increase in the overall length can be avoided in a simple way asa result of the designs of regulating cylinders suitable both for theantilock control and for the slip/propulsion control.

Using a restoring spring and a specific minimum stroke of the mastercylinder pistons or from a minimum pedal travel, makes an additionalcontribution to the restoring force counteracting the actuating force ofthe master cylinders. Thus, it is possible in a simple way to obtain abetter approximation of the installed front-axle/rear axle division ofbrake pressure to its ideal pattern and, to that extent, also animprovement in the braking decelerations obtainable, without loss ofdriving stability.

The space enclosed by the additional restoring spring which isappropriately designed as a helical spring, can be utilized toaccommodate an electrical displacement sensor for generating outputsignals which are characteristic of the position of the master cylinderpisto of a brake circuit which can be subjected to the control. Theseposition output signals in combination with the output signals fromwheel-speed sensors monitoring the movement behavior of the brakedand/or driven vehicle wheels, can be used to obtain the most effectivepossible control of pressure build-up and pressure reduction phases ofthe antilock control or of the slip/propulsion control.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematical cross-sectional view of a first exemplaryembodiment of a brake unit according to the invention which has ahydraulic brake booster and which comprises two static master cylinders,a drive cylinder and a compensating cylinder for eliminating the idletravel of the pistons of the master cylinders, in a section along thecommon plane of the central axes of the master cylinder and of the brakebooster.

FIG. 2 is a sectional view of a further exemplary embodiment of a brakeunit according to the invention in a representation truncated along thecentral axis of the brake booster, but otherwise corresponding to thatof FIG. 1.

FIG. 3 is a schematic cutaway of an exemplary embodiment of a brake unitaccording to the invention with opposed pistons which can be used forantilock control and are assigned to the master cylinder one of thebrake circuits of the vehicle and which, when subjected to pressure,cause the master cylinder piston to experience a displacement in thedirection of its basic position, in a representation truncated along thecentral axis of the brake booster, but otherwise corresponding to thatof FIG. 1.

FIG. 4 is a cutaway view of a further exemplary embodiment of a brakeunit according to the invention with an opposed piston which can be usedfor antilock control, in a representation according to that of FIG. 3.

FIGS. 5 and 6 are cutaway views showing details of the master cylinder,assigned to the brake circuit of the driven vehicle wheels, of a brakeunit according to FIGS. 1 or 2, with devices both for antilock controland for propulsion control, in a representation corresponding to FIGS. 3and 4.

FIG. 7 is a cutaway view of a modification of the exemplary embodimentaccording to FIG. 6 with an additional spring for influencing theinstalled brake pressure distribution at high brake pressure.

FIG. 8 is a cutaway view of a modification of the brake unit accordingto FIG. 6, in that the regulating cylinder for the propulsion control isarranged laterally relative to the master cylinder.

FIG. 9 is a cutaway view of a further exemplary embodiment of a brakeunit with a double-acting regulating cylinder for antilock andpropulsion control of the driven vehicle axle, in a representationcorresponding to FIGS. 3 to 8.

FIG. 9a is a view of the brake unit according to FIG. 9 in the directionof the arrow IX, to explain the arrangement of the master cylinders andregulating cylinders for the antilock and propulsion control.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a brake unit 10, according to the invention, for aroad vehicle with a hydraulic dual-circuit brake system which comprisesa front-axle brake circuit I and a rear axle brake circuit II.

These brake circuits I and II are represented in FIG. 1 by thediagrammatically indicated front-wheel brakes 11 and 12, rear wheelbrakes 13 and 14 and main brake lines 16 and 17 of the front-axle brakecircuit I and rear axle brake circuit II, respectively, which lead fromthe brake unit 10 to these wheel brakes 11 to 14.

The brake circuits I and II are designed as so-called static brakecircuits which are each subjected to pressure, within the framework ofthe brake unit 10, by a static master cylinder 18 and 19.

Within the housing 23 of the brake unit 10, the so-called twinarrangement is chosen for these master cylinders 18 and 19, so that themaster cylinders 18 and 19 are arranged at a lateral distance next toone another, with their central longitudinal axes 21 and 22 parallel toone another. Thus, a considerable reduction of the overall length of thebrake unit 10, as measured in the direction of these longitudinal axes21 and 22 is achieved as compared with an otherwise conventional tandemarrangement.

Also incorporated in the brake unit 10 is a hydraulic brake booster 24,comprising a hydraulic drive cylinder 26, and brake valve 32. Hydraulicdrive cylinder 26, when subjected to pressure, causes the pistons 27 and28 of the master cylinders 18 and 17 to be drivable for the purpose of apressure build-up in their outlet-pressure spaces 29 and 31. The brakevalve 32 feeds a control pressure, proportional to the actuation forceof the brake pedal 33 for the build-up of brake pressure, into the drivepressure space 34 of the drive cylinder 26 to cause the hydraulic brakeboosting desired. Drive cylinder 26 is illustrated as a hydraulic linearmotor arranged between the master cylinders 18 and 19, with its centrallongitudinal axis 36 parallel to and coplanar with the centrallongitudinal axes 21 and 22.

The housing bores 37 and 38, in which the pistons 27 and 28 of themaster cylinders 18 and 19 are arranged displaceably, are limitedfixedly by the end-face wall 39, on the left according to FIG. 1, of thebrake-unit housing 23. The pistons 27 and 28 which form the axiallymovable limitations of the outlet-pressure spaces 29 and 31 of themaster cylinders 18 and 19, each have axial spaced two piston flanges41, 42 and 43, 44 which are sealed off from the housing bores 37 and 38by annular gaskets 46, 47 and 48, 49. An annular follow-up space 51, 52is limited, within the respective housing bore 37, 38, by the pistonflanges 41, 42 and the piston flanges 43, 44 of the piston 27 and 28,respectively. The follow-up spaces 51 and 52 are constantly connected tothe brake-fluid storage tank 56 of the brake system via a follow-upchannels 53 and 54, respectively.

The annular gaskets 46 and 48, between follow-up spaces 51 and 52 andoutlet-pressure spaces 29 and 31 of the respective master cylinders 18or 19, are designated as lip gaskets which, in interaction with overflowbores, not shown for the sake of simplicity, which pass axially throughthe piston flanges 41, 43, also performs a function of check valves. Ifnecessary during return movements of the pistons 27 and 28 taking placewith the effect of a reduction of pressure in the outlet-pressure spaces29 and 31, these check valves allow brake fluid to overflow from thefollow-up spaces 51 and 52 of the master cylinders 18 and 19 into theiroutlet-pressure spaces 29 and 31.

The master cylinder pistons 27 and 38 are urged by restoring springs 57,58 into the basic position shown, corresponding to the non-actuatedstate of the brake system. These restoring springs 57, 58 can be clampedwithin the cylinder bore 37 between the end-face wall 39 and the piston27 as shown for spring 57 or external the bore 31 as shown for spring58.

Compensating flow paths or snifting bores 61 and 62 are opened in thebasic position or basic positions of the pistons 27 and 28 of the mastercylinders 18 and 19 and via which brake fluid can flow from thebrake-fluid storage tank 56 into the outlet-pressure spaces 29 and 31 orback from these to the brake-fluid storage tank. Upon brake systemactuation, and a displacement of the master cylinder pistons 27 and 18in the direction of the arrow 66 to effect a build-up of brake pressure,the mouth orifices of snifting bores 61 and 62 are crossed by therespective annular gasket 46 and 48, and consequently theoutlet-pressure spaces 29 and 31 are shut off from the brake-fluidstorage tank 56. In the course of the further displacement of thepistons 27 and 28, in the direction of the arrow 66, the build-up ofbrake pressure in the outlet-pressure spaces 29 and 31 of the brake unit10 takes place. It goes without saying that, instead of such sniftingbores 61 and 62, it is also possible to provide, on the master cylinderpistons 27 and 28, central valves of known design which, in the basicposition of the pistons, are held in their open position, in which theoutlet-pressure space 29 or 31 of the master cylinder 18 or 19 is incommunication with the follow-up space 51 or 52. As soon as the pistons27 and 28 have executed an initial portion of their stroke correspondingto the closing travel of such valves, the control valves move into theblocking position, from which any further displacement of the pistons 27and 28 in the direction of the arrow 66 is linked to a pressure build-upin the particular outlet-pressure space 29 or 31.

Starting from the basic position shown, the pistons 27 and 28 of themaster cylinders 18 and 19 must, at all events, execute an initial "idletravel" before a pressure build-up proportional to the pedal travelstarts in the outlet-pressure spaces 29 and 31 of the master cylinders18 and 19. The minimum value of this idle travel is determined by theinitial proportion of the particular piston stroke which must beexecuted before the snifting bores 61 and 62 are shut off from therespective outlet-pressure spaces 29 and 31.

The drive cylinder 26 of the brake booster 24, which generates theactuating force exerted on the pistons 27 and 28 of the master cylinders18 and 19, is designated as a single-acting linear cylinder. A piston 67of drive cylinder 26 is guided displaceably in a central bore 68 of thehousing 23 in the direction of the central longitudinal axis 36 of thiscentral bore 68 and is sealed off from the latter. The piston 67 can bedisplaced in the direction of the arrow 66 by the outlet pressure of thebrake valve 32 proportional to the pedal force or to the pedal travel.

The control bore 68 receiving the drive piston 67 has, in the axialdirection, the same extent as the bores 37 and 38 of the mastercylinders 18 and 19, and is closed off on the side on the left accordingto FIG. 1, by a portion of the end-face wall 39 of the housing 23.

A compensating space 69, limited fixedly relative to the housing by thisportion of the end-face wall 39 and movably by the piston 67 and locatedin the drive cylinder 26, is connected to the brake-fluid storage tank56.

The piston 67 of the drive cylinder 26 is sealed off from the centralbore 68 by two annular gaskets 70, 71 which are fixed to flanges of thepiston 67 and which are axially spaced from one another by a distancesomewhat greater than the maximum stroke of the piston 67, alsocorresponding to the maximum strokes of the master cylinder pistons 27and 28. An inlet pressure space 72, formed by a shallow annular groovein piston 67, between flanges of the piston 67 which receive the annulargaskets 70 and 71, is connected, via a housing channel 73, to thehigh-pressure outlet of a hydraulic auxiliary-pressure source, shown asa pressure accumulator 74, of the brake booster 24. In the illustratedbasic position of the drive-cylinder piston 67, corresponding to thenon-actuated state of the brake system, the housing channel 73 opens outimmediately next to the left-hand flange 76 of the drive piston 67, thatcarries the sealing ring 70 delimiting the pressureless compensatingspace 69 from the inlet-pressure space 72. Thus, the communicatingconnection between the inlet-pressure space 72 and theauxiliary-pressure source 74 is preserved within the possible stroke ofthe drive piston 67.

The drive piston 67 has a central continuous longitudinal bore 77 whichwidens in a step-shaped manner towards the drive pressure space 34.Guided displaceably in a pressure tight manner in the bore step 77' ofleast diameter, extending between the compensating space 69 and thesomewhat wider middle bore step 77", is a valve piston 78 of the brakevalve 32. The valve piston 78 is of elongate pot-shaped design havingbottom 79 closing off the smallest bore step 77' from the compensatingspace 69 in a pressure tight manner. The end portion of the narrowestbore step 77' which receives the piston bottom 79 of the valve body 78extends within a short piston extension 81 which projects into thecompensating space 69. The outside diameter of the piston extension 81is clearly less than that of the adjacent piston flange 76 which issealed off from the central housing bore 68 by the annular gasket 70.

The piston extension 81 is provided with a compensating channel 82 whichtakes the form of a transverse bore and connects the compensating space69 with portion 77' of the central bore 77 of the piston 67 which issealed off by the piston bottom 79. Within that portion of the drivepiston 67 between the annular gaskets 70 and 71, there is a secondtransverse channel 83 which opens, at an axial distance from thecompensating channel 82 into the narrowest bore portion 77' of thecentral bore 77 of the piston 67 and is connected in a communicatingmanner to the annular-gap-shaped inlet-pressure space 72 of the brakebooster 24.

Within the bore step 77"' of largest diameter of the drive piston 67, anelongate tubular piston rod 84 is connected fixedly to the drive piston67. The outside diameter of piston rod 84 is somewhat less than thediameter of the widest bore step 77"' of the drive piston 67 and theinside diameter of which corresponds to the diameter of the middle borestep 77" of the drive piston 67. This piston rod 84 is sealed off fromthe widest bore step 77"' of the drive piston 67 by an annular gasket86. The piston rod 84 passes through mutually aligned central bores 87of a partition-wall 89, which forms a limitation, fixed relative to thehousing, of the drive pressure space 34, limited movably by the drivepiston 67, and central bore 88 through a housing cover 91 which closesoff the housing 23 of the brake unit 10 relative to the outside on thesame side as the pedal. The length of end portion 92 of the piston rod84 which projects from the housing 23, 91 in the illustrated basicposition of drive piston 67 corresponds at least to the maximum strokeof the drive piston 67, so that the piston rod 84 remains guided in thecentral bore of the housing cover 91 in every operating state of thebrake system.

An actuating piston 93 is guided displaceably within that end portion 92of the piston rod 84 at least in the basic position shown, and is sealedoff from the elongate cylindrical hollow space 96 of the piston rod 84which is open towards the central bore 77 of the drive piston 67 by anannular gasket 94 arranged fixedly relative to the piston. The actuatingpiston 93 is connected fixedly in terms of displacement to the valvepiston 78 of the brake valve 32 by an elongate push rod 97. Possiblerelative movements of the composite piston structure, comprising theactuating piston 93, the push rod 97 and the brake-valve piston 78,relative to the piston rod 84 are limited by stop flange 98 of actuatingpiston 93. In the direction of the arrow 66, the stop flange 98 abuts astop step 99 of the piston rod 84 and, in the opposite directionrepresented by the arrow 101, the stop flange 98 abuts a stop ring 102inserted into the end portion 92 of the piston rod 84. The axialdistance between the annular stop face 103 of this stop ring 102 and thestop step 99 of the piston-rod portion 92 is selected just large enoughto ensure that the necessary brake-pressure and boosting controlfunctions can be performed within the relative movement stroke of thevalve body 78 in relation to the drive piston 67 made possible thereby.

The brake pedal 33 is connected positively and non-positively to theactuating piston 93 by a ball-and-socket joint connection via a tappet104, and can be actuated counter to the restoring force of a pathsimulation spring 107 supported on the housing cover 91 and on a tappetflange 106. The tubular piston rod 84 is sealed off, by an annulargasket 108 and designed as a lip gasket, both from the central housingbore 68 and from the central bore 87 of the partition-wall part 89, onwhich the annular gasket 108 is supported in the axial direction. Asseen in the axial direction, a traverse bore 109 is provided in thepiston rod 84 between this annular gasket 108 and the annular gasket 86sealing off the inner end portion of the piston rod from the largestbore step 77"' of the drive piston 67. The drive pressure space 34 ofthe drive cylinder 26 communicates with the interior 96 of the pistonrod 84 via transverse bore 109 and thereby also with the middle borestep 77" and that part of the larger bore step 77"' of the drive piston67 which communicates with step 77".

An overflow orifice 112 is provided in that portion of the brake valvepiston 78 casing 111 projecting into the middle bore step 77" of thedrive piston 67. The axial extent and arrangement of overflow orifice112 is such that, in any of the possible positions of the valve piston78 in relation to the drive piston 67, the interior 113 of the valvepiston 78 is in communication with the middle bore step 77" of the drivepiston 67 and, via the transverse bore 109 of the piston rod 84, alsowith the drive pressure space 34 of the drive cylinder 26.

The valve piston 78, in the part of its casing 111 immediately adjacentto the piston bottom 79, is provided with a first outer annular groove114 which communicates with the interior 113 of the valve piston 78 viaa transverse bore 116. In the illustrated basic position of the valvepiston 78 of the brake valve 32 and of the drive piston 67 of the brakebooster 24, there is partial overlapping of the inner mouth orifice ofthe compensating channel 82 of the piston 67 with the first annulargroove 114 of the valve piston 78. Thus, in this basic position of thebrake valve, the drive pressure space 34 of the drive cylinder 26communicates with the compensating space 69 of the brake booster 4. Agroove flank 117 of the annular groove 114, which is on the inside, asseen in the axial direction, and which is on the right according to FIG.1, extends within the mouth orifice of the compensating channel 82 andforms a control edge. Upon actuation of the brake system and theresulting displacement of the valve piston 78 in the direction of thearrow 66, the control edge crosses the mouth of the compensating channel82, shutting off the compensating space 69 from the interior 113 of thevalve piston 78 and consequently also from the drive pressure space 34of the drive cylinder 26.

The valve piston 78 is provided with a second outer annular groove 118which likewise communicates with the interior 113 of the valve piston 78via a transverse bore 119.

This second annular groove 118 is so arranged between the first annulargroove 114 and the overflow orifice 112, as seen in the axial direction,that its outer groove flank 121, on the left according to FIG. 1, entersthe clear cross-sectional region of the inner mouth orifice of thetransverse channel 83 connecting the inlet-pressure space 72 to thecentral bore 77 of the drive piston 67. Thereby high outlet pressure ofthe auxiliary-pressure source 74 is fed into the drive pressure space 34of the drive cylinder 26 only when and preferably precisely when theinner control edge 117 of the first outer annular groove 114 hascompletely crossed and therefore closed the inner mouth orifice of thecompensating channel 82 of the drive piston 67.

As a result of this, pressure acting on the annular surface of drivepiston 67 limits the drive pressure space 34 and has an effective valveF₁ of the surface difference F₂ -F₃ between the cross-sectional surfaceF₂ of the central bore 68 and the total cross-sectional surface F₃ ofthe piston rod 84, a force exerted in the direction of the arrow 66 andensuring brake boosting acts on the drive piston 67. This force istransmitted to master cylinder pistons 27 and 28 via a rocker designatedas a whole by 122, coupled in terms of movement in the axial directionto the drive piston 67 or piston rod 84 and pivotable about an axis 123extending at right angles to the central longitudinal axis 36 of thepiston rod 84 and at a lateral distance from the central longitudinalaxis 36, and via push rods 124 and 126 which, in the arrangementillustrated, are supported between the rocker arms 127 and 128 and onthe master cylinder pistons 27 and 28. Thus, the master cylinder pistons27 and 28 are displaced to effect a build-up of brake pressure in theoutlet-pressure spaces 29 and 31 of the master cylinders 18 and 19.

The actuating piston 93 is also subjected to the outlet pressure, fedinto the drive pressure space 34, of the brake valve 32, the pressureoutlet of which is formed by the overflow orifice 112, on an annularsurface, the amount f₁ of which is determined by the difference f₂ -f₃between the cross-sectional surface f₂ of that part of the centralhollow space of the piston rod 84 receiving the actuating piston 93 andthe cross-sectional surface f₃ of the smallest bore step 77' of thecentral bore 77 of the drive piston 67. A force directed in thedirection of the arrow 101, that is to say in the opposite direction tothe pedal force, is thereby exerted on the actuating piston 93 and, whenit exceeds the pedal force, results in a displacement of the valvepiston 78 in the direction of the arrow 101 in relation to the drivepiston 67. The inlet-pressure space 72 thereby is shut off from thedrive pressure space 34 once again, and instead the drive pressure space34 is once more connected to the compensating space 69 of the brakebooster. As a result of this interaction of the brake valve 32 and ofthe drive cylinder 26 of the brake booster 24, a pressure proportionalto the pedal force is always fed into the drive pressure space 34 anddesired brake boosting is consequently achieved.

The dimensions of the cross-sectional surfaces F_(V) of the housing bore37 of the master cylinder 18 for the front-axle brake circuit I andF_(H) of the housing bore 38 of the master cylinder 19 for the rear axlebrake circuit II are such that, for the same displacement travels of themaster cylinder pistons 27 and 28 in the outlet-pressure spaces 29 and31 of the master cylinders 18 and 19, the same pressure is built up.Also, the surface ratio F_(V) /F_(H) corresponds to the brake pressuredistribution B_(VA) /B_(HA) arrived at as a fixed setting, B_(VA)denoting the front-axle brake pressure fraction and B_(HA) denoting therear axle brake pressure fraction.

The ratio L_(V) /L_(H) of the effective lengths L_(V) and L_(H),measured from the pivot axis 123 of the rocker 122 to the centrallongitudinal axes 21 and 22 of the master cylinders 18 and 19 or of thepush rods 124 and 126, of the rocker arms 127 and 128, via which thedrive force generated by the drive cylinder 26 is transmitted to themaster cylinder pistons 27 and 28, corresponds to the ratio F_(H)/F_(V). This ensures that when the brake system is actuated, the pistons27 and 28 of the master cylinders 18 and 19 are displaced uniformly inthe direction of the arrow 66, that is to say the rocker 122 maintainsits orientation, shown in FIG. 1, at right angles to the piston rod 84.The push rods 124 and 126 are connected, via ball-and-socket joints, tothe rocker 122 and to the respective pistons 27 and 28 of the mastercylinders 18 and 19.

In order largely to avoid "spreads" of the pedal characteristic of thebrake system caused by production tolerances of the master cylinderpistons 27 and 28, the housing 23 and/or the brake valve 32, that is tosay in order to keep the spread range, within which the brake pressurelinked to a specific actuating travel of the pedal can vary, as narrowas possible and consequently achieve a sharply defined response behaviorof the brake unit 10 and of the brake system of the vehicle as a whole,there is provided a compensating cylinder 129. In the introductory phaseof brake actuation by increased pressure being fed into the drivepressure space 34 of the drive cylinder 26 via the brake valve 32, thecompensating cylinder 129 is likewise subjected to this pressure. Thiscauses a limited introductory displacement of the rocker 122 and of thepistons 27 and 28 of the master cylinders 18 and 19 in the direction ofthe arrow 66, even before the drive piston 67 and the connected pistonrod 84 have experienced a displacement in the direction of the arrow 66.

As a result of this introductory displacement of the master cylinderpistons 27 and 28, they initially assume an advanced position, in whichthe outlet-pressure spaces 29 and 31 of the master cylinders 18 and 19are already shut off from the snifting bores 61 and 62. That is to saythe pistons 27 and 28 have executed their above-described "idletravels", within which no brake pressure can be built up. With adisplacement of the drive piston 67 resulting from the pressure in thedrive pressure space 34 of the drive cylinder 26 and with the furtherresulting displacement of the master cylinder pistons 27 and 28, thesedisplacements being proportional to the pedal force and to the pedaltravel, a pressure build-up taking place in virtually strict proportionto the pedal travel or to the pedal actuation force is now obtained inthe master cylinder outlet-pressure spaces 29 and 31.

The compensating cylinder 129 comprises a cylindrical pot-shaped housing131 which, as seen in the basic position illustrated, is for the mostpart received by a likewise cylindrical and pot-shaped bulge 132,pointing towards the brake pedal 33, of the housing cover 190. Thebottom 133 of the housing part 131 is arranged immediately next to thebottom part of this bulge 132, and the annular end face 134 of thehousing casing 135 points towards the rocker 122 or to the drivecylinder 26. The housing 131 is connected to the piston rod 84 whichpasses through a central bore of the housing bottom 133 and which issealed off from this central bore by an annular gasket 137. Furthermore,the compensating cylinder 129 comprises an annular piston 138surrounding the piston rod 84 coaxially over a portion of its length andhaving a central bore 139, through which the piston rod 84 passes. Theannular end face 141 of annular piston 138 facing the housing bottom 133is equipped with an annular gasket 142 which, in the exemplaryembodiment illustrated, is designed as a lip gasket and which seals offthe piston 138 both relative to the piston rod 84 and relative to thecasing 136 of the housing 131. The piston 138 is guided displaceablyboth on the piston rod 84 and on the inner cylindrical surface of thehousing casing 136.

Fastened to the piston rod 84 between the drive cylinder 26 and thecompensating cylinder 129 is a supporting and stop flange 143. Acompression spring 144 supported on flange 143 urges the piston 138 intoits illustrated basic position which is marked by the abutting of aannular flange 146 of the piston against the annular end face 134 of thecasing 136 of the housing 131. The compression spring 144 is received,over most of its length, in a recess 147 of the piston 138. Annulargasket 142 and piston 138 form the movable axial limitation of a furtherannular operating-pressure space 148 which is limited fixedly relativelyto the housing by the bottom 133 of the housing 131. Operating pressurespace 148 is connected via a transverse bore 149 of the piston rod 84,to the interior 96 of the piston rod 84 and consequently to thebrake-valve outlet 112.

The rocker 122 is designed, in its central region, as a frame 151 whichsurrounds the piston 138 and the compensating-cylinder housing 131 at aradial distance and which, being pivotable about the axis 123, isconnected in an articulated manner to the outer flange 146 of the piston138.

The annular surface 141 of the piston 138, on which the pressure whichcan fed into the further drive pressure space 148 is exerted, is, forexample, 20% larger than the annular surface of the drive piston 67limiting the drive pressure space 34 of the drive cylinder 26. When thedrive pressure space 148 of the compensating cylinder 129 is subjectedto pressure as a result of the actuation of the brake pedal 33, itspiston 138 experiences a relative displacement in relation to the pistonrod 84 in the direction of the arrow 66. This relative displacement islimited as a result of the abutting of the piston 138 against the stopflange 143 of the piston rod 84. The resulting displacement of themaster cylinder pistons 27 and 28 of the master cylinders 18 and 19 ofthe brake unit 10 is sufficient for a brake pressure build-up of a fewbars, to which the wheel brakes 11 to 14 are subjected when braking isinitiated. After which, with a further increase in the pedal force, anapproximately linear relationship between the pedal travel and the brakepressure is obtained.

In the event of a failure of an auxiliary-pressure source 74, brakeboosting ceases. However, brake pressure can be built up in both brakecircuits I and II solely by a pedal force which, as soon as the stopflange 98 of the actuating piston 93 comes up against the stop step 99of the piston rod 84, is transmitted to the piston rod 84 andconsequently also to the rocker and, via this, to the pistons 27 and 28of the master cylinders 18 and 19. Since, in the event of a failure ofthe auxiliary-pressure source 74, the force directed counter to thepedal force and resulting from pressure exerted on the annular surfacef_(l) of the drive piston ceases, the pedal force can be utilized fullyfor the build-up of the brake pressure. Thus, when the pedal force isused for the brake-pressure build-up in the event of a failure of theauxiliary-pressure source, a "transmission-ratio jump" takes effect andcompensates some of the loss of brake pressure caused by the absence ofbrake boosting.

In the event of a failure of one of the two brake circuits I or II, theother brake circuit continues to remain operational because the pivotingangle of the rocker 122 is limited as a result of a stop effect.

In this case, an idle travel and consequently a lengthening of the pedaltravel first occur, until the rocker is halted as a result of a stopeffect and brake pressure can be built-up in the still operational brakecircuit.

The brake unit 20 illustrated as a further exemplary embodiment in FIG.2, is identical to the brake unit 10 according to FIG. 1 in terms offunction. The same reference symbols as in FIG. 1 are chosen to denoteelements of a brake unit 20 which are or can be identical in terms ofdesign to corresponding functional elements of the brake unit 10according to FIG. 1, and to that extent reference is made to therelevant parts of the description for the sake of simplicity.

Elements of the brake unit 20 shown in FIG. 2 which are similar in termsof function to functional elements of the brake unit 10 according toFIG. 1 are likewise designated by ten reference symbols which are usedin FIG. 1, but which are additional given an "'".

This method of designation is also maintained for the other drawingFIGS. 3-9a.

The representation of FIG. 2 is restricted essentially to thosefunctional elements which differ in constructional terms from thecorresponding functional elements of the brake unit 10 according to FIG.1, in particular the brake booster 24', its drive cylinder 26', thebrake valve 32' and the compensating cylinder 129'.

In so far as elements of brake units according to the inventionillustrated in FIG. 2 and further drawing Figures bear the samereference symbols as in FIG. 1, reference will therefore always be madeto the parts of the description which belong to this.

In the brake unit 20 according to FIG. 2, the housing 152 of the drivecylinder 26' is designed as an elongate cylindrical pot-shaped part ofthe housing cover 91' closing off the housing 23' of the brake unit 20on the same side as the pedal. This housing part 152, extending alongthe central longitudinal axis 36 of the brake booster 24' and of thehousing 23', projects into a likewise pot-shaped cylindrical hollowspace 153 of the housing 23' which extends between the master cylinders18 and 19 and which is closed off by the end-face wall 39 of the housing23' on the side located opposite the brake pedal.

The bore 68' receiving the drive piston 67' and located in the drivecylinder housing 152 is closed, on the same side as the pedal, by aclosing piece 154 which is sealed off from this bore by 68' by anannular gasket 108'. The annular gasket 108' forms the limitation, fixedrelative to the housing, of the drive pressure space 34', which islimited movably by the drive piston 67', of the drive cylinder 26' ofthe brake booster 24'. At the end of the drive-cylinder housing 152distant from the pedal 33 and on the left according to FIG. 2, thedrive-cylinder housing 152 is closed off by an end-face wall 157 whichforms the limitation, fixed relative to the housing, of the compensatingspace 69', limited movably by the drive piston 67', of the drivecylinder 26'. The flanges of the drive piston 67' which are equippedwith the annular gaskets 70 and 71 limit, in the axial direction, theinlet-pressure space 72 which permanently communicates with theauxiliary-pressure source 74 via the housing channel 73. As a result ofthe pedal-controlled actuation of the brake valve 32', inlet pressurespace 72 can be brought into communicating connection with the drivepressure space 34' of the drive cylinder 26' and with a further drivepressure space 148' of the compensating cylinder 129' of the brake unit20. In the brake unit 20 of FIG. 2, the piston rod 84' corresponding infunctional terms to the piston rod 84 of brake unit 10 according to FIG.1 projects from the side of the drive piston 67' limiting thecompensating space 69' and passes through a central bore 158 in theend-face wall 157 of the cylinder housing 152. The piston rod 84' issealed off from the cylinder housing 152 by an annular gasket 159 fixedto the housing.

Once again, the piston rod 84' is elongate and in the form of a hollowtube. Its interior 96 communicates via the compensating channel 82located immediately next to the flange 76', on the left according toFIG. 2, of the drive piston 67', with the compensating space 69' whichitself is connected to the storage tank 56 of the brake system via ahousing channel 161. On a free end portion 162 of the piston rod 84'protecting from the cylinder housing 152 on the end face is arranged aflange 131' in the form of an annular disc, which is sealed off from thefree end portion 162 of the piston rod 84' by an inner annular gasket163 and is displaceable together with the piston rod 84'. Flange 131'corresponds in functional terms to the compensating-cylinder housing 131of the brake unit 10 according to FIG. 1.

This flange 131' constitutes the inner axial limitation of the drivepressure space 148' of the compensating cylinder 129', its outer axiallimitation, on the left according to FIG. 2, being formed by the bottompart 164 of a generally pot-shaped piston 138' which corresponds infunctional terms to the annular piston 138 of the brake unit 10according to FIG. 1. The flange 131' is sealed off by an outer annulargasket 166 from the inner cylindrical surface 167 of the casing 168 ofthe pot-shaped piston 138'. The pot-shaped piston 138' has a centralslender rod-shaped inner extension 169 which extends over most of thelength of the interior 96 of the piston rod 84' and to the inner end ofwhich, namely that facing the drive piston 67', is attached a radialsupporting flange 171.

A restoring spring 144' extends between the supporting flange 171 and athrust ring 172 inserted into the piston rod 84'. The thrust bearing 172is supported in the axial direction on an annular step surface 173formed between the central longitudinal bore 174 of the piston rod 84'adjacent to an of the same diameter as the central bore 77 of the drivepiston 67' and a bore portion 176 of slightly smaller diameter whichpasses axially through the free end portion 162 of the piston rod. Therestoring spring 144', in the exemplary embodiment illustrated, isdesigned as a helical compression spring coaxially surrounding therod-shaped extension of the pot-shaped piston 138'. Counter to therestoring force of restoring spring 144', the pot-shaped piston 138' isdisplaceabl relative to the piston rod 84' or to the flange 131' when,during actuation of the brake system, an increased outlet pressure ofthe brake valve 32' is fed into the drive pressure space 148' of thecompensating cylinder 129'.

The stroke of the relative movements possible between the pot-shapedpiston 138' and the piston rod 84' of the drive piston 67' of the drivecylinder 26' is limited as a result of abutting a stop ring 177,arranged on the inner face of the pot casing 168, against the outer edgeof the flange 131'. To that extent, stop ring 177 is similar in terms offunction to the stop flange 143 of the piston rod 84 of the brake unit10 according to FIG. 1.

The valve piston 78' of the brake valve 32' is guided displaceably andin a pressure tight manner in the central bore 77 of the drive piston77' and in the central bore 174 continuing this and located in thepiston rod 84'. The valve piston 78' is likewise cylindrical and tubularand is connected fixedly in terms of pulling and pushing to an actuatingpiston 93', on which acts, via the pedal tappet 104, the pedal forceexerted on the brake pedal 33 when the brake system is actuated. Theactuating piston 93' is guided displaceably in a central bore 178 of thehousing closing part 154 and is sealed off relative to the central bore178 of the housing closing part 154 and relative to the drive pressurespace 34' of the drive cylinder 26' by an annular gasket 179. Within thedrive pressure space 34', the composite piston structure consisting ofthe valve piston 78' and of the actuating piston 93' is equipped with athrust ring 180 connected firmly to the actuating piston 93'.

Between the thrust ring 180 and an annular surface 181, movably limitingthe drive pressure space 34' on one side of the drive piston 67' of thedrive cylinder 26' extends a helical compression spring 107' which isunder moderate prestress and which is similar in functional terms to thepath simulation spring 107 of the brake unit 10 according to FIG. 1.

The drive pressure space 34' of the drive cylinder 26' is in constantcommunication connection with the interior 113 of the valve piston 78'via an overflow orifice 112 which is designated as a transverse borelocated in a region of the valve piston 78' having an outside diameterless than the inside diameter of the central bore 77 of the drive piston67'. The end-face edge 183 of the valve piston 78' facing the piston rod84' forms the control edge. When the brake unit 20 is actuated, thecontrol edge 183 crosses over the compensating channel 82 and shuts offthe compensating space 69' from the interior 96 of the piston rod 84',from the interior 113 of the valve piston 78', and consequently alsofrom the drive pressure space 34' of the drive cylinder 26'.

The valve piston 78' is provided with an outer annular groove 118 whichcommunicates with the interior 113 of the valve piston 78' via atransverse bore 119. A central flange 184 separates the annular groove118 from the portion 185 of the valve piston 78' which is connectedfixedly to the actuating piston 93'. The outside diameter of portion 185is somewhat less than the diameter of the central bore 77 of the drivepiston 67'. An outer groove flank 121, on the left according to FIG. 2,of the annular groove 118 forms a control edge. As soon as the controledge 183 has crossed over and thereby closed the compensating channel 82when the brake system is actuated, the outer groove flank 121 enters themouth region of the transverse channel 83 connecting the inlet-pressurespace 72 to the central bore 77 of the drive piston 67'. This results inan increased pressure derived from the auxiliary-pressure source 74being fed both into the drive pressure space 34' of the drive cylinder26' and into the drive pressure space 148' of the compensating cylinder129'. Because of the above-described design of the brake valve 32' whichcorresponds in functional terms to the brake valve 32 of the brake unit10 according to FIG. 1, this pressure is proportional to the actuatingforce exerted on the brake pedal 33.

The pot-shaped piston 138' is supported on the bottom part 186 of apot-shaped sleeve 187. The casing 188 of the sleeve 187, as seen in thebasic position shown, surrounds the housing 152 of the drive cylinder26' coaxially over virtually its entire length and is guideddisplaceably thereon in the axial direction. The rocker 122 of the brakeunit 20 is connected to the pot-shaped sleeve 187 in an articulatedmanner, so as to be pivotable about the axis 123 extendingperpendicularly to the drawing plane.

The restoring springs 57 and 58 engage on the pistons 27 and 28 of themaster cylinders 18 and 19, and urge the master cylinders 27 and 128,the pot-shaped sleeve 187, the pot-shaped piston 138', supported onthis, of the compensating cylinder 219' and the piston 67' of the drivecylinder 26' into the basic position illustrated. The valve piston 78'and, together with this, the brake pedal 33' are urged into the basicposition by the path simulation spring 107'.

So that the housing channels 73 and 161 can be kept advantageouslyshort, they are guided to a connection piece 189 which is connectedfirmly both to the part of the housing 23' limiting the central hollowspace 153 and to the elongate cylindrical housing part 152 extendingwithin the pot-shaped sleeve 187. To allow the necessary displacementmovements of the sleeve 187 relative to the housing 23' in the directionof the arrow 66, the casing 188 of the sleeve 187 is provided with alongitudinal slot 191, through which the connection piece 189 passesradially.

In contrast to the representation of FIG. 2 selected for explanatorypurposes, the housing channels 73 and 161 and the connection piece 189are appropriately arranged such that the central axes of the housingchannels 73 and 161 in the central axes of the connecting channels ofthe connection piece 189 which communicate with them are in a planeextending perpendicularly to the drawing plane.

The closing piece 154 has an external thread 193 meshing with aninternal thread 192 of the housing cover 91'. By rotating the closingpiece 154, it is possible to shift it axially and consequently adjustthe control edges 183 and 121 of the valve piston 78' exactly relativeto the mouth orifices of the compensating channel 82 and of thetransverse channel 83 of the piston 67'. The closing piece 154 can befixed by a lock nut 190 in the position corresponding to the idealsetting of the control edges 183 and 121.

In the brake unit 20 too, the pistons 27 and 28 of the master cylinders18 and 19 experience a minimum displacement with the effect of abrake-pressure build-up, as soon as an increased outlet pressure of thebrake valve 32' is fed into the drive pressure spaces 148' and 24' ofthe compensating cylinder 129' and of the drive cylinder 26' during anintroductory actuation of the brake pedal 33. This results in thesensitive response behavior, already explained above, of the brakesystem as a whole and leads to an essentially linear pedalcharacteristic even at only low actuating forces. In the event offailure of the auxiliary-pressure source 74, the counter force whichotherwise results when the actuating piston 93' is subjected to pressureon its surface corresponding to the cross-section of the bore 178ceases, so that the pedal force takes full effect on the drive piston67' as soon as the actuating piston 93' runs with its stop face 195 ontothe drive piston 67'.

A series of special designs which, when used within the framework of abrake unit 10 or 12, as described with reference to FIGS. 1 and 2, makeit possible to perform additional functions, in particular a simpleantilock control on the rear axle (FIGS. 3 to 9) and a simplestarting-slip control (FIGS. 5 to 9), are explained below with referenceto FIGS. 3 to 9, for the master cylinder assigned to the brake circuitII of the driven vehicle wheels, namely the rear wheels in theexplanatory example chosen.

In the master cylinder part 19 of a brake unit 10 or 20, shown in FIG. 3and assigned to the rear axle brake circuit II, on both sides of thehousing bore 38 receiving the master cylinder piston 28 there are twoblind bores 194 and 196. Each blind bore 194, 196 receive an opposedpiston 197, 198 forming the axially movable limitation of respectivecounterpressure space 199, 201 which is limited fixedly relative to thehousing by the respective inner end-face wall 202, 203 of the particularblind bore 194, 196. The opposed pistons 197 and 198 are made elongateand pot-shaped, and in the basic position shown, corresponding to thenon-actuated state of the brake piston. Their bottom parts 204, 206limiting the counterpressure spaces 199, 201, are arranged in theimmediate vicinity of the end-face walls 202 and 203 of the bores 194and 196 and are sealed off relative to these bores by a lip gasket 207fixed to the piston, as provided for the piston 197 shown in the upperpart of FIG. 3, or by an annular gasket 208, as provided for the opposedpiston 198 shown in the lower part of FIG. 3.

The piston 28 of the master cylinder 29 is equipped with a tubularextension 209 which points and is open towards the arm 128 of the rocker122. The tubular extension 209 coaxially surrounds the push rod 126, viawhich the rocker 122 engages on the piston 28. The outside diameter ofthe tubular extension 209 is at most equal to the diameter of the mastercylinder bore 38 and is preferably somewhat less than this. At the endof the tubular extension 209 facing the rocker arm 208, the extension209 is equipped with a radially projecting supporting flange 211.Between the bottom parts 204 and 206 of the opposed pistons 197 and 198and the supporting flange 211 of the piston extension 209 extendrestoring springs 58 which urge the master cylinder piston 28 and rocker122, on the one hand, and the opposed pistons 197 and 198, on the otherhand, into the illustrated basic position marked by a stop effect. Theserestoring springs 58 are centered within the opposed pistons 197 and 198by the inner cylindrical surfaces of the pistons and outside them bystop rods 212 which are connected fixedly to the supporting flange 211and which, as seen in the basic position illustrated, extend a short wayinto the opposed piston 197 and 198. When the piston 28 has executed itsmaximum pressure build-up stroke, the entire length of the stop rods 212has entered the opposed pistons 197 and 198.

If during a braking operation, a tendency of locking occurs on one ofthe rear wheels of the vehicle (the sensor and the processing devicesnecessary for detecting this can be presumed to be known), a solenoidvalve 213 is commanded to move out of its basic zero position shown intoits excited position I. In the basic zero position, the counterpressurespaces 199 and 201 are connected to the brake fluid storage tank and areshut off from the auxiliary pressure source 74. In the excited positionI, the counterpressure spaces 199 and 201 are connected to the auxiliarypressure source 74 and are shut off from the brake fluid storage tank orthe tank of the auxiliary pressure source. As a result, the opposedpistons 197 and 198 are forced in the direction of the arrow 214, to theright according to FIG. 3. When the piston bottom parts 204 and 206 runup against the free ends of the stop rods 212, and the master cylinderpiston 28 is taken up so as to execute a pressure reducing movement. Apressure drop results from the enlargement of volume of the outletpressure space 31 of the master cylinder 18 and counteracts the tendencyto locking in the connected wheel brakes.

Because of the pivotable mounting of the rocker 122, a pressure drop inthe rear axle brake circuit II only is possible, without any appreciablereaction on the front axle brake circuit. The central axes 214 and 216of the opposed piston 197 and 198 are symmetrical in relation to thecentral axis 22 of the master cylinder 19 and are arranged so that theylie in a common plane with this, this plane preferably beingperpendicular to the sectional plane selected for the illustration, incontrast to the representation chosen for explanatory purposes. Becauseof the relatively small surface annular gaskets 207 and 208, thefrictional resistances, counter to which the displacement of the opposedpistons 197 and 198 takes place, are correspondingly low.

In the exemplary embodiment according to FIG. 4, the part of the housing23 assigned to the master cylinder 19 for the rear axle brake circuit IIis provided with a deep annular groove which coaxially surrounds thecentral bore 38 in which the piston 28 is guided displaceably in apressure tight manner, and which is open on the side pointing towardsthe rocker 122. The piston 28, including the extension part 209 and thesupporting flange 211, corresponds in terms of its construction to thepiston 28 according to FIG. 3. Inserted in the annular groove 217 is acylindrical sleeve 218 which extends virtually over the entire axialdepth of the annular groove 217. In the immediate vicinity of the bottom219 of the annular groove 217, the cylindrical sleeve 18 is equippedwith an annular flange 220 pointing radially inwards which is sealed offfrom the outer cylindrical surface of the casing region 222 of thehousing 23 directly surrounding the piston 28 by an annular gasket 221.The inner annular gap remaining between the sleeve 218 and the casingregion 222 receives, over some of its length, a restoring spring 58which is supported, on the same side as the housing, on the flange 220of the sleeve 218 and, on the same side as the piston, on the flangeportion 211. The restoring spring 58, urges the piston 28 into its basicposition shown, corresponding to the non-actuated state of the brakesystem.

A thick-walled opposed piston 226, in the form of a cylindrical shell,is guided displaceably in the radially outer annular gap 224 remainingbetween the sleeve 218 and the outer cylindrical limiting surface of theannular groove 217. The opposed piston 226 is sealed off from the sleeve218, and the outer limiting wall 224 of the annular groove 217 by anannular gasket 227 which, in the exemplary embodiment illustrated, isdesigned as a lip gasket and which is arranged on the end face of theopposed piston 226 facing the groove bottom 219. As already describedwith reference to FIG. 3 with respect to the counterpressure spaces 199and 201 illustrated there, when the counterpressure space 228, limitedmovably in the axial direction by the opposed piston 226 and fixedlyrelative to the housing by the groove bottom 219, is subjected topressure under valve control when a tendency to locking occurs on one ofthe wheels of the circuit II during a braking operation, the opposedpiston 226 can be displaced in the direction of the arrow 215. As soonas the piston 226 runs up against the supporting flange 211, the piston28 of the master cylinder 19 is displaced with the effect of a pressurereduction in the brake circuit II. The elements of a brake unit 10 or 20which are shown in FIG. 4 are completely identical in functional termsto the elements shown in FIG. 3.

The elements of the brake unit according to FIG. 4 which are necessaryin order to achieve the antilock control function are characterized bytheir constructively simple and rotationally symmetrical design.

Under the exemplary embodiment illustrated in FIG. 5, the part of amaster cylinder 19' which is assigned to the rear axle brake circuit IIis of a design similar to that of the master cylinder 19 according toFIG. 3, but which differs from this, in that the restoring spring 58which urges the master cylinder piston 28' into its basic positionshown, is one again arranged within the outlet pressure space 31 of themaster cylinder 19'. Another difference is that a piston 228, of adouble-acting regulating cylinder 229 is connected to the mastercylinder piston 28'. The two working spaces 232 and 233, delimitedmovably from one another in the axial direction by a flange 231 of thepiston 228 of the double acting regulating cylinder 229 are alternatelysubjected to pressure and relieved of pressure. With the brake systemactuated, the piston 228 is displaceable in the direction of the arrow215 counter to the hydraulically boosted actuating force with the effectof a pressure reduction in the outlet pressure space 31 of the mastercylinder 19'. Without the brake system being actuated, the piston 228 isdisplaceable with the effect of a pressure build-up in the outletpressure space 31 of the master cylinder 19', that is to say in thedirection of the arrow 66.

The displacement of the master cylinder piston 28' in the direction ofthe arrow 215, necessary in order to stop a tendency to locking on oneor both of the vehicle wheels of the rear axle brake circuit II, isobtained by subjecting the working space 232 to pressure. As alreadyexplained with reference to FIG. 3, this is achieved by activating asolenoid valve 213 by a pressure reduction control signal from anelectronic control unit (not shown) of the antilock device.

To control the drive-slip control function (ASC function), there is asecond solenoid valve 234 which is designed as a 2/3-way valve. In thebasic position 0, a second working space 233 of the double-actingregulating cylinder 229 is connected to the brake fluid storage tank 56,and is shut off from the auxiliary pressure source 74. In the excitedposition I, where the second solenoid valve 234 is controlled by anoutput signal from an electronic control unit (not shown) of the ASCcontrol device, when a tendency to spinning occurs on at least one ofthe driven vehicle wheels, the auxiliary pressure source 74 of the brakebooster 24 or 24' is connected to the second working space 233, and isshut off from the brake fluid storage tank 56.

The piston 228 of the double-acting regulating cylinder 229 is designedas a cylindrical sleeve, the casing 236 of which is connected fixedly tothe supporting flange 211 projecting radially from the end of the pistonextension 209. The piston 228 is guided displaceably by casing 236, onthe circular-cylindrical outer surface 237 of the inner casing region222 of the housing 23 surrounding the piston 28' of the master cylinder19'. The piston 228 is sealed off from this outer surface 237 by anannular gasket 238 fixed to the housing and arranged on that end portionof this inner casing region 222 of the housing 23 which is located onthe same side as the rocker.

A flange 231 projects radially outwards from the inner edge of thecasing 236 and into the annular groove coaxially surrounding the housingcasing 222. The flange 231 is sealed off from the other casing surface241 of a cylindrical sleeve 242 by an annular gasket 239 fixed to thepiston. The cylindrical sleeve 242 is inserted into the annular groove217 and is sealed off from the cylindrical housing surface 243 limitingthe annular groove 217 on the outside. The cylindrical sleeve 242 isequipped, on its outer end portion facing the rocker 122, with a flange244 pointing radially inwards, which is sealed off from the radiallyouter casing surface 247 of the casing 236 of the piston 228 of thedouble-acting regulating cylinder 229 by an annular gasket 246.

The operating space 233 in the form of an annular gap is limited fixedlyrelative to the housing by flange 244 of the sleeve 242 and is limitedmovably by the radial flange 231 of the piston 228 which causes thedrive-slip control on the rear axle. As a result of the feeding of theoutlet pressure of the auxiliary pressure source 74 into the workingspace 232 in the form of an annular gap, extending between the bottom ofthe groove 217 and the flange 231 of the piston 228 of the double actingregulating cylinder 229, if required the antilock control function ofthe brake unit can be achieved on the rear axle of the vehicle. In thedesign of a brake unit 10 or 20 shown in FIG. 5, altogether aspace-saving coaxial arrangement of the elements necessary for antilockand drive-slip control is obtained.

The special exemplary embodiments, illustrated in FIGS. 6 to 8, differfrom the exemplary embodiment according to FIG. 4, in that they includeregulating members for achieving a starting-slip control or propulsioncontrol, but include a sleeve-shaped opposed piston 226 provided forachieving the antilock control function of FIG. 4. To the extentindicated by the use of identical respective reference symbols,reference may be made to the description relating to FIG. 4 and thedescription of the exemplary embodiments according to FIGS. 6 to 8 canbe restricted to their additional functional elements.

In the exemplary embodiment according to FIG. 6, the regulating cylinderprovided for the starting-slip control function (ASC function) is asingle-acting regulating cylinder 248. The piston 249 of cylinder 248can be supported, via an elongate bar-shaped push rod 251, on theoutside, facing the rocker 122, of the supporting flange 211 of thepiston 28 of the master cylinder 19 assigned to the rear axle brakecircuit II. The housing 252 of the ASC-regulating cylinder 248 isdesigned as a cylindrical pot-shaped rear widening of the housing cover91 of the housing 23 of the brake unit 10 or 20 and is closed off on theoutside by the end-face wall 253. In the basic position 0 of the ASCcontrol valve 234, the drive pressure space 254, closed off movably bythe piston 249 and fixedly relative to the housing in the axialdirection by this end-face wall 253, is shut off from the auxiliarypressure source 74 and is connected to its tank or to the brake fluidstorage tank 56 of the brake system. As long as this occurs, the piston249 is urged, by a restoring spring 256 surrounding the push rod 251coaxially and supported on a guide block 257 fixed to the housing, intothe basic position shown, in which the piston 249 is supported on theend-face wall 253 via a short spacer piece. The push rod 251 passesthrough a guide bore 258 in the guide block 257 and, as seen in theillustrated basic position of the various functional elements, its freeend 259 is located at a short distance of 1 to 2 mm from the supportingflange 211 of the piston 28 of the master cylinder 19.

In order to activate the ASC control, the control valve 234 is switchedto its excited position I, wherein the drive pressure space 254 isconnected to the auxiliary-pressure space 74 and is shut off from itstank or from the brake fluid storage tank 56. The displacement of theASC-regulating piston 249 in the direction of the arrow 66 istransmitted to the master cylinder piston 28 via the push rod 251 whichengages on the supporting flange 211 of the master cylinder piston 28 ata short radial distance from the central longitudinal axis 22 of themaster cylinder 19. This results in the brake-pressure build-upnecessary for ASC control taking place in the outlet pressure space 31of the master cylinder 19.

As already described with reference to FIG. 5, the antilock control isactivated by switching the ABS control valve 213 from its basic position0 into the excited position I, in which the outlet pressure of theauxiliary pressure source 74 is fed into the working space 232.

The exemplary embodiment illustrated in FIG. 2 differs once again fromthat according to FIG. 6 only in additional cushioning and supportingelements, by which, from a specific minimum outlet pressure in theoutlet pressure space 31 of the master cylinder 19, a variation in thebrake pressure distribution to the front axle and to the rear axle isachieved with the effect of reducing the rear axle brake pressurefraction.

A description of the exemplary embodiment according to FIG. 7 istherefore restricted to these additional elements and otherwisereference is made to the description of FIG. 6, with the referencesymbols used there being given.

In contrast to the representation of FIG. 7 chosen for the simplicity,the housing 23 is provided with two cylindrical pot-shaped widenedportions 261, with their central longitudinal axes 262 extendingparallel to the central longitudinal axis 22 of the master cylinder 19,and arranged symmetrically in relation to the central longitudinal axis22 of the master cylinder 19, in such a way that their common plane isperpendicular to the drawing plane. Arranged in each of these lateralwidened portions is an additional restoring spring 263 which, at oneend, is fastened, in a way not shown in detail, to that end-wall portion264 of the housing 23 which forms the bottom of the pot-shaped widenedportion 261. The spring 263 is designed as a helical spring which isarranged axially relative to the longitudinal axis 262. The free end 266of the spring 263 is located at a distance from the supporting flange211 of the latter corresponding to between 30% and 60% of the maximumstroke which the piston 28 can execute between its basic position andits end position linked to a maximum outlet pressure of the mastercylinder 19.

When the brake is actuated, the piston 28 of the master cylinder 19 isdisplaced until the supporting wings 267 of the supporting flange 211run up against the spring ends 266 and the restoring springs 263 arethereby compressed. From this piston position onwards, the restoringforce exerted on the piston 28 increases to a greater extent than ifthere were only the restoring spring 58. Because of this increase in therestoring force exerted on the piston 28 and the transmission of thehydraulically boosted actuating force to the master cylinder pistons 27and 28 by the rocker 122 pivotable about the axis 123, as soon as aspecific minimum pedal travel or a minimum pedal force is exceeded,there is a reduction of the rear axle brake pressure fraction inrelation to the front axle brake pressure fraction. This is advantageousin terms of a stable driving and braking behavior of the vehicle, withgood braking deceleration nevertheless being achieved at the same time.

The space remaining within the coils of the restoring spring 236 can beutilized to accommodate a displacement sensor 268 designed, for example,as a slide potentiometer, which comprises a resistance element connectedfirmly to the housing 23 and a pick-up element displaceable togetherwith the supporting flange 211. The displacement sensor 268 generates anoutput signal which is characteristic of the distance of the mastercylinder piston 28 from its basic position. The amplitude and timebehavior of this output signal contains information which can be usedfor an appropriate control of antilock and/or ASC operations, forexample for controlling the rebuild-up of pressure after a pressurereduction phase of the antilock control.

The exemplary embodiment according to FIG. 8 differs from that accordingto FIG. 6 only in the arrangement provided for the regulating cylinders248' necessary for ASC control.

In contrast to the representation of FIG. 8, there are two regulatingcylinders 248' which are arranged symmetrically relative to the centralaxis 22 of the master cylinder 19 assigned to the rear axle brakecircuit II and of which the central axes 269 extending parallel to thecentral longitudinal axis 22 of the master cylinder 19 are in a planeperpendicular to the drawing plane and containing the central axis 22 ofthe master cylinder 19. The housing bores 171, within which the pistons272 of the ASC-regulating cylinders 248' are guided displaceably in apressure tight manner, are arranged laterally next to the annular groove217 receiving the opposed pistons 226 for the antilock control and areclosed off by a portion 273 of the housing end wall.

On the side facing the rocker 122 or the supporting flange 211 of themaster cylinder piston 28, the housing bores 271 of the ASC-regulatingcylinders 248' are each closed off by a guide bushing 274 insertedfirmly into the respective bore. Passing through a central guide bore276 in said bushing 274, is a slender piston 277 which is connectedfirmly to the piston 272 and which is connected fixedly in terms oftension to lateral supporting wings 267' of the supporting flange 211 ofthe master cylinder piston 28. The supporting wings 267' are eachprovided with an orifice 278 which is coaxial relative to the centrallongitudinal axis 269 of the particular regulating cylinder 248' andthrough which passes the free end portion 279 of the piston rod 277. Astop ring 281 is fixed to the piston rod on the outside of therespective supporting wing 267' of the supporting flange 211 and therebyensures that the piston 272 of the regulating cylinder 248' is coupledfixedly, in terms of tension, to the piston 28 of the master cylinder19.

In the exemplary embodiment illustrated, an annular gasket 282 isdesigned as a lip gasket and seals off the piston rod 277 of the piston272 from the central bore 276 of the guide bushing 274 and from thehousing bore 271 of the regulating cylinder 248'. The guide bushing 274and annular gasket 282 form the limitation, fixed relative to thehousing, of an annular drive pressure space 283 which is separated fromthe compensating space 284 of the regulating cylinder 248' by the piston272.

Irrespective of actuation of the brake pedal 33, when the outletpressure of the auxiliary pressure source 76 is fed into the drivepressure space 283 under solenoid valve control, the brake pressurebuild-up necessary for an ASC control phase is consequently possible inthe outlet pressure space 31 of the master cylinder 19, the piston rods277 acting as tension rods. During a braking operation, the pistons 272of the regulating cylinders 248' remain in their basic position shown asa result of the effect of restoring springs 286. Inasmuch as referencesymbols given in FIG. 8 are not mentioned in the part of the descriptionbelonging directly to FIG. 8, attention will therefore be drawn to thedescription of the parts of FIG. 6 which are designated as identical.

The exemplary embodiment of a brake unit 10 or 20 according to theinvention illustrated in FIG. 9 is functionally equivalent to thataccording to FIG. 8. It differs from this from the point of view ofconstruction, only in that instead of a single regulating cylindersurrounding the opposed piston 226, for the antilock control, and twosingle-acting regulating cylinders 248' for the ASC control, there arenow two double-acting regulating cylinders 287 which are used both forantilock control and for propulsion or ASC control.

In contrast to the representation of FIG. 9, chosen for the sake ofsimplicity, the two regulating cylinders 287 are arranged in such a waythat their central axes 262 extend in a plane which contains the centralaxis 22 of the master cylinder 19 and which is perpendicular to thedrawing plane. As can be seen from the representation in FIG. 9a, thisis a plane 288 which forms an angle of approximately 70° with thedrawing plane 289 in the representation of FIG. 9a. Restoring springs 58which urge the piston 28 of the master cylinder 19 into the basicposition shown, corresponding to the non-actuated state of the brakesystem or brake unit 10 or 20, are supported between the housing 23 andthe supporting flange 211 of the master cylinder piston 28. Therestoring springs 58 are received, over some of their length, in blindbores 291 in the housing 23. The central longitudinal axes 292 of blindbores 291 extend at a lateral distance from the central longitudinalaxes 262 of the double-acting regulating cylinders 287, in thearrangement illustrated in FIG. 9a and is symmetrical relative to thecentral longitudinal axis 22 of the master cylinder 19. The arrangementof the master cylinder 19 within the housing 23, of the regulatingcylinders 287 and of the blind bores 291 for the restoring springs 58results in a compact space-saving overall design, as seen in thecross-sectional representation of FIG. 9a.

In the same way as described with reference to FIG. 8, the piston 272',separating the working space 232' for the antilock control from thedrive pressure space 283' for the ASC control within the bore 271' ofthe regulating cylinder 287, is connected to the supporting flange 211of the piston 28 of the master cylinder 19 fixedly in terms of tensionvia an elongate piston rod 277'. The subjection of either one of theworking spaces 232' and 283' to pressure for the activation of theantilock control or the ASC control respectively is controlled in thesame way as explained with reference to FIGS. 5 and 6.

Where the antilock control is concerned, a driving-back force acting inthe direction of the arrow 215 is exerted on the piston 28 of the mastercylinder 19, as soon as the piston 272' of the regulating cylinder 287runs with a stop shoulder 293 of its piston rod 277' up against thesupporting flange 211 of the piston 28.

Possible relative movements between the piston 28 of the master cylinder19 and the piston 272' of the regulating cylinder 287 are limited to amaximum stroke which corresponds to the distance existing between thestop shoulder 293 and the supporting flange 211 in the basic positionshown.

In the illustrated basic position of the piston 272' of the regulatingcylinder 287, the axial distance of the flange of the piston 272'separating the working spaces 232' and 283' relative to one another fromthe guide bushing 274 is also equal to this maximum stroke of thepossible relative movements or somewhat greater than this.

The piston 272' is equipped with a central extension 294 in the form ofa push rod, projecting into the working space 232' and surroundedcoaxially by a helical spring 296 which urges a thrust ring 297, axiallydisplaceable relative to the push rod 294, up against an end stop 298 ofthe push rod 294. During normal braking, the working or drive pressurespaces 232' and 283' of the regulating cylinder 287 are keptpressureless.

As soon as, in the event of a displacement in the pressure build-updirection 66 of the piston 28 supported by its supporting flange 211 onthe stop shoulder 293 of the regulating-cylinder piston 272', the stopring 297 comes up against a stop shoulder 299 of the bore 271' in thehousing of the regulating cylinder 287, any further displacement of thecomposite piston structure 28, 272' also takes place counter to the nowincreasing restoring force of the spring 296. The effect is similar tothe effect of the spring 163 of the brake unit 10 or 20 according toFIG. 7.

Although the present invention has been described and illustrated indetail, it is to be clearly understood that the same is by way ofillustration and example only, and is not to be taken by way oflimitation. The spirit and scope of the present invention are to belimited only by the terms of the appended claims.

What is claimed:
 1. Brake unit with a hydraulic brake booster and anantilock system for a road vehicle with a hydraulic dual circuit brakesystem having a front-axle/rear-axle division of the brake circuitscomprising:a pair of master cylinders each assigned to one of the twobrake circuits, said master cylinders being arranged in a twin design ina brake unit housing, and master pistons of the master cylinders beingactuable via a torque-compensated rocker; the brake booster has, as aboosting element, a drive cylinder, a drive piston which forms theaxially movable limitation of a drive pressure space which, when thebrake system is actuated, can be subject to an auxiliary pressure froman auxiliary pressure source proportional to brake pedal force, therocker being coupled in terms of movement to the drive piston and beingpivotable about an axis extending perpendicularly to the longitudinalaxis of the drive piston; in the event of failure of an auxiliarypressure source, the drive piston of the drive cylinder and, togetherwith this, the rocker are displaceable solely by a pedal force which canbe transmitted to the drive piston via a tappet; the antilock systemworks, in at least one of the two brake circuits, on the principle ofreducing the brake pressure by enlarging the volume of the outletpressure space of that master cylinder connected to the wheel brake orwheel brakes which can be subjected to control; and the master cylinderof the brake circuit which can be subjected to control has a countercylinder having an opposed piston which can be supported on the masterpiston of this master cylinder and which, when subjected on one side tothe outlet pressure of the auxiliary pressure source of the brakebooster, causes the master piston to be displaceable in the direction ofits basic position counter to the actuating force taking effect on this.2. Brake unit according to claim 1, including a regulating cylinder insaid brake unit housing having a regulating piston which acts on themaster piston of the master cylinder of the brake circuit assigned tothe drive vehicle wheels and which, when subjected on one side to theoutlet pressure of the auxiliary pressure source of the brake boostercauses the master piston to be displaceable with the effect of a brakepressure build-up in its outlet pressure space, independently of anactuation of the brake system.
 3. Brake unit according to claim 2,wherein the counter-cylinder used for the antilock control and theregulating cylinder which can be used for a slip or propulsion controlare combined into a double-acting hydraulic counter, the working pistonof which separates the working spaces which can be alternately subjectedto pressure and relieved of pressure.
 4. Brake unit according to claim1, wherein the master piston of the master cylinder of the brake circuitwhich can be subjected to control has a tubular extension which pointstowards the rocker and which includes, at its free end facing therocker, a supporting flange, on which engage the regulating membersprovided for the antilock control.
 5. Brake unit according to claim 1,wherein for antilock control there are at least two counter-cylinderswhich are arranged in an axially symmetrical grouping relative to thecentral longitudinal axis of the master cylinder within the housing ofthe brake unit.
 6. Brake unit according to claim 5, wherein thecounter-cylinders provided for the antilock control have elongatepot-shaped pistons which are open towards the rocker and on the bottomparts of which are supported restoring springs which urge the masterpiston into its basic position and which, at the other end, engage on asupporting flange fixed to the drive piston.
 7. Brake unit according toclaim 1, wherein the opposed piston is a cylindrical sleeve displaceablein a pressure-tight manner in an annular gap of the housing coaxiallysurrounding the drive cylinder.
 8. Brake unit according to claim 7,wherein the annular gap receiving the opposed piston is formed by theradially outer cylindrical limiting surface of an annular groove of thehousing and the outer casing surface of a sleeve, which is inserted in apressure-tight manner into this annular groove and which is sealed offfrom the surface limiting the annular groove on the inside and, togetherwith this surface, forms an inner annular gap, in which a restoringspring, urging the master piston of the master cylinder into its basicposition, is received over some of its length.
 9. Brake unit accordingto claim 2, wherein the housing of the regulating cylinder is acylindrical pot-shaped widening of the housing cover closing off thehousing of the brake unit on the same side as the pedal, and theregulating piston, received in the bore of the housing, of theregulating cylinder engages via a tappet on a supporting flange of themaster piston or on the arm of the rocker supported on this.
 10. Brakeunit according to claim 2, wherein for slip control there are at leasttwo regulating cylinders, the regulating pistons of which are receivedin housing bores which are grouped around the central longitudinal axisof the master cylinder in an axially symmetrical arrangement relative tothis and which are closed off, on the same side as the pedal, by guidebushings with guide bores, through which piston rods of the regulatingpistons pass displaceably in a pressure-tight manner, said piston rodspassing through orifices, aligned with the guide bores, in thesupporting flange located at the end of an extension of the masterpiston on the same side as the pedal, and each being supportable by astop on the outside, facing the pedal, of the supporting flange. 11.Brake unit according to claim 10, wherein the regulating cylinders aredouble-acting cylinders, and the piston rods passing through theorifices in the supporting flange of the master piston are equipped withstop shoulders which are supportable on the inside, facing the piston ofthe regulating cylinders, of the supporting flange connected fixedly tothe master piston.
 12. Brake unit according to claim 1, including arestoring spring urging the master cylinder piston into its basicposition, and at least one additional restoring spring which, after aminimum stroke of the master cylinder piston during braking, makes anadditional contribution to the restoring force taking effect on thepiston.
 13. Brake unit according to claim 12, wherein the additionalrestoring spring is arranged in a cylindrical pot-shaped widening, opentowards the pedal side and arranged laterally relative to the mastercylinder bore and relative to a groove coaxially surrounding this, ofthe housing of the brake unit and is fastened on one side to theend-face wall remote from the pedal, a free end portion of theadditional restoring spring being located opposite a supporting flangewhich is connected fixedly to the extension of the drive piston locatedon the same side as the pedal and of which the axial distance from thefree end of the additional restoring spring, as seen in the basicposition of the piston, is between one third and one half of the maximumdrive piston stroke.
 14. Brake unit according to claim 1, including adisplacement sensor in said housing for generating an electrical outputsignal characteristic of the position of the master piston.